1. Field of the Invention
The present invention relates to a method for preparing cadmium sulfide nanocrystals emitting light at multiple wavelengths, and cadmium sulfide nanocrystals prepared by the method. More particularly, the present invention relates to a method for preparing cadmium sulfide nanocrystals by mixing a cadmium precursor and a dispersant in a solvent that weakly coordinates to the cadmium precursor, heating the mixture to obtain a cadmium precursor solution, dissolving a sulfur precursor in a solvent that weakly coordinates to the sulfur precursor to obtain a sulfur precursor solution, feeding the sulfur precursor solution to the heated cadmium precursor solution maintained at a high temperature to prepare cadmium sulfide crystals, and growing the cadmium sulfide crystals; and cadmium sulfide nanocrystals prepared by the method. The cadmium sulfide nanocrystals thus prepared can emit light close to white light at different wavelengths corresponding to transitions from various energy levels.
2. Description of the Related Art
When a semiconductor material is prepared into nano-sized crystals (nanocrystals), quantum confinement effects are exhibited in the range smaller than the bulk exciton Bohr radius of the semiconductor material. Due to quantum confinement effects, the band gap energies characteristic to the semiconductor material are changed. When a semiconductor material capable of emitting visible light is prepared into nanocrystals, the band gap energies of the semiconductor material begin to increase as the semiconductor material decreases in size down to a particular size, which results in a blue shift wherein the luminescent region is shifted toward the blue region is observed. Since the control over the characteristics, structure, shape, and size of a quantum dot material enables the control of the corresponding band gaps, energy levels over a very broad range of wavelengths can be obtained.
In this connection, recent attention has been directed toward techniques for controlling the growth of quantum dots in order to develop new concepts of semiconductor devices. In particular, conventional vapor deposition processes, including MOCVD (metal organic chemical vapor deposition) and MBE (molecular beam epitaxy), are suitable for use in controlling semiconductor thin films to a level corresponding to a single atomic layer and controlling the growth of quantum dots. However, since quantum dots prepared by vapor deposition using the phenomenon of lattice mismatch have serious drawbacks in the control of their size, uniformity, density, etc., despite their good crystallinity, it is known to one skilled in the art that vapor deposition processes are inappropriate for the fabrication of commercially viable devices.
U.S. Pat. No. 6,225,198 discloses a process for preparing semiconductor quantum dots using a Group II-VI compound. According to this process, the semiconductor quantum dots are prepared by dissolving a material containing a Group II metal (e.g., Zn, Cd, or Hg) and a material containing a Group VI element (e.g., S, Se, or Te), as precursors, in first and second dispersant solutions, respectively, mixing the solutions, adding a solvent capable of dissolving both the precursors to the mixture, preparing Group II-VI compound semiconductor crystals while maintaining a temperature for crystal growth, growing the Group II-VI compound semiconductor crystals to a desired size, and separating the grown compound semiconductor crystals. The nanocrystals thus prepared have different energy band gaps according to their size, due to quantum confinement effects. Particularly, nanocrystals with inorganic material characteristics emit pure light in the corresponding energy region.
U.S. Pat. No. 6,306,736 discloses a process for preparing Group II-VI semiconductor quantum dots by utilizing the process disclosed in U.S. Pat. No. 6,225,198.
Core-shell structured quantum dot materials exhibiting improved luminescence efficiency, and a method for preparing the quantum dot materials are disclosed in U.S. Pat. Nos. 6,322,901 and 6,207,229, respectively. The core-shell structured quantum dots are reported to have improved luminescence efficiency by 30˜50%. The prior art techniques state that the quantum dots are of interest for use in displays and biological imaging sensors, based on the phenomenon that energy transitions in quantum dots mainly take place at the edge of energy band gaps. According to the prior art, in order to produce white light at different wavelengths, various luminescent nanocrystals having a different size or composed of materials emitting light over a broad range of wavelengths should be mixed, followed by photoluminescence. Thus, there exists a strong need for a method for preparing nanocrystals capable of emitting light over a broad range of wavelengths (at multiple wavelengths), without mixing of various nanocrystals emitting light at different wavelengths.